1. (Field of the Invention)
The present invention relates to a constant current source circuit, and more particularly, to a constant current source circuit formed on a compound semiconductor substrate such as a semi-insulating gallium arsenide (GaAs) substrate.
2. (Description of Related Art)
As shown in FIG. 1A, a conventional constant current source circuit of this kind has the circuit construction wherein the source (S) and gate (G) of a field effect transistor (hereinafter called as FET) 31 are connected to the same power voltage line 310 which is connected to the terminal TP.sub.31, the gate-source voltage V.sub.GS of this FET 31 is fixed to 0 V and the drain (D) is connected to and used as a current output node TO.sub.31. Alternatively, as shown in FIG. 1B, a resistor element R.sub.31 is inserted between the source (S) of an FET 32 and the power voltage line 310, a predetermined reference voltage V.sub.RR, which is generated internally or supplied from an external power source, is applied to the gate (G) of this FET 32 and the drain (D) of the FET 32 is connected to and used as the current output node TO.sub.32.
The conventional constant current source circuit described above, however, has the construction in which the gate-source voltage V.sub.GS is set to 0 V or a predetermined fixed voltage V.sub.RR is applied to the gate. Therefore, if the threshold voltage V.sub.T of each FET 31, 32 deviates from the center of the design value due to the fabrication condition or the like, the supply current deviates from a design value, too, so that the noise margin of a logic circuit or an output circuit connected to this constant current source circuit drops or the output level deviates greatly from the design value.